Headset receiver



July 26, 1955 M. L. ToUGER` 5T AL 2,714,134

HEADSET RECEIVER Filed Feb. 27. 195] 2 Sheets-Sheet 1 Hy. Z.

la j; .2 L70 l WMU? 42g' c 1 'UWZWHLV 1?- lllv 1 /125766 T4258 lNvEN-roRS AT'oRNEY July 26, 1955 Filed Feb. 27, 195] M. L. TOUGER ET AL HEADSET RECEIVER 2 Sheets-Sheet 2 ATTORNEY i United States Patent ilce 2,714,11i Patented July 26, 1955 HEADSET RECEIVER Martin L. Touger, Audubon, and Alfred H. Kettler, Collingswood, N. J., assignors, by mesne assignments, to the United States of America as represented by the Secretary of the Air Force Application February 27, 1951, Serial No. 213,022

7 Claims. (Cl. 179-1155) The present invention relates to sound translating apparatus and more particularly to a headset receiver of the type useful in iluid mediums having relatively great differences in density.

Although the requirements for successfully operating sound translating devices, such as headset receivers, commonly referred to as earphones, in mediums having relatively small dilerences in density are satisfactorily met by conventional instruments in use today, these instruments are found to be unsuitable for operations in mediums having relatively great differences in density. In the eld of aviation, for example, communication equipment must operate efficiently between air densities at sea level and 40,000 feet altitude. Tests of various receivers presently in use show considerable variation in output level and frequency response with relatively large differences in density of the medium in which they are operated, hence they are unsuitable for aviation purposes.

Attempts have been made to solve this problem, for example, by the use of electrical switches for increasing the electrical power to the headset receiver. The switches are usually connected to a pressure-sensitive device in a manner such that they will automatically compensate for decreases in level of response as a consequence of decreases in pressure of the fluid medium in which the instrument is being used. This type of control, however, does not provide frequency response correction without further use of corrective filters. As a result thereof, undesirable resonances, which at normal pressures would not be annoying, are amplied to an extent such as to be painful to the ear of a user of the instrument. Another type of earphone or headset receiver which has been proposed is that disclosed in the U. S. patent to L. L. Beranek, 2,505,519. This receiver is an attempt to manipulate the spacing of the magnetic elements to correct sensitivity as well as partially compensate for frequency response by physically changing the acoustic constants. This apparatus has been found to be objectionable from the standpoint of mechanical complexity. Still another type of instrument has been proposed to circumvent this problem by designing a conventional receiver for an air density which is the mean of two extreme densities of a lluid medium in which the apparatus is to be used. The objection to this type of earphone is that the variations in sensitivity and frequency response are merely reduced somewhat in magnitude.

It is a primary object of the present invention to provide a sound translating device which will operate etliciently in a fluid medium of widely varying density.

It is another object of the present invention to provide a sound translating device which will have substantially uniform output level when operated in a fluid medium of widely varying density.

It is also an object of the present invention to provide a headset telephone receiver for aviation use which will have the highest practical sensitivity when operated at different altitudes in a fluid medium having wide variations in density.

Still another object of the present invention is to provide a headset telephone receiver which will automatically compensate for changes in density of the fluid medium in which it is operated without substantial changes in sensitivity of the receiver.

A further object of the present invention is to provide a sound translating device the sensitivity of which is substantially uniform and unaffected by wide variations in density of the uid medium in which it is operated.

Another object of the present invention is to provide a headset telephone receiver which is simple in construction and highly eicient in use.

In accordance with the present invention, an improved vibratory system is provided for a dynamic sound translating device. The vibratory system comprises an acoustically closed cavity provided behind the vibratory member or diaphragm of the device which functions as a compensating acoustic impedance. The dimensions of the cavity are such that the reflected acoustic stiffness of the cavity bears a predetermined relation to the acoustic stiffness of the diaphragm such as to provide a minimum sensitivity change with changes in density of the fluid medium in which the device is being used. Pressure equalizing means, which is impervious to audio frequency sound waves, is provided for elfecting a balance of pressure on opposite sides of the diaphragm.

The novel features characteristic of the present invention, as well as additional objects and advantages thereof, will be better understood from the following detailed description when read in connection with the accompanying drawing in which,

Figure l is a front elevation of a headset receiver, in accordance with the present invention.

Figure 2 is a sectional view of the receiver shown in Figure l, taken on the line 2 2 of Figure 1,

Figure 3 is a plan view of the seal ring of the receiver,

Figure 4 is an equivalent acoustical circuit diagram for an earphone coupled to an ear of a user,

Figure 5 is an equivalent electrical circuit diagram for a dynamic earphone coupled to an ear of a user,

Figure 6 is a family of curves showing the sensitivity loss of an acoustic stiffness controlled dynamic earphone as a function of the ratio of reflected acoustic stiffness to diaphragm stilfness,

Figure 7 is an equivalent electrical circuit diagram of the earphone illustrated in Figures 1 and 2,

Figure 8 is a set of curves showing the frequency response of the earphone illustrated in Figures 1 and 2, taken at densities corresponding to sea level and 40,000 feet altitude pressures, and

Figure 9 is a set of curves similar to the curves shown in Figure 8, taken for a conventional type earphone without compensation for sensitivity changes.

Before referring to the accompanying drawings in greater detail, it is desired to point out the background theory leading to the development of the present invention. For the purposes of the present invention, the dynamic earphone is employed since it is found to be the most feasible for offering improvements in response over other types of earphones in use at the present time. For convenience, the units used in all equations or expressions are measured according to the C. G. S. system.

As stated above, earphones of the present day design show considerable variation in output level, that is sensitivity, with relatively great `variations in density of the fluid medium in which they are used. There are two aspects to the acoustical consideration for eliminating loss in sensitivity with altitude, namely: (1) the stiffness of the cavity behind the diaphragm may be increased until the refiected acoustic stiffness (where the stiffness of the cavity behind the diaphragm affects diaphragm movement as a function of the square of the diaphragm area) controls diaphragm motion; (for example, by decreasing the cavity volume); and (2) the area of the diaphragm may be increased until `the ear cavity stiffness, as referred to the diaphragm, controls diaphragm motion. For each of these two aspects, the diaphragm stiffness must bear a proper relation to the reflected acoustic stiffness. 'The present invention is concerned with emphasis on the first aspect, namely, control of diaphragm motion by the cavity stiffness behind the diaphragm.

At frequencies below 4000 cycles the acoustic cavity comprising the ear cavity Vand the space defined by a close fitting earcap for the receiver may be considered, to a first approximation, about 6 Vcubic centimeters in volume. Consequently, an earphone placed over the ear, in such a manner as to prevent sound leakage between the earcap of the earphone and the head of a user, may be represented by a pressure generator having certain internal acoustical impedance feeding an acoutical stiffness load.

The acoustical stiffness of the ear cavity thus provided is shown by the expression:

Ve (l) Where (p) :the air density (C )=the velocity of sound, considered constant independent of air density; and,

Ver-the volume of ear cavity including the space defined by a close fitting earcap.

The expression for the pressure developed in the ear cavity is:

where: =the R. M. S. value of the sound wave volume displacement (the sound wave volume displacement being equal in magnitude to the diaphragm lineal displacement multiplied by the diaphragm effective area);

In order to maintain the sound pressure in the ear cavity constant and independent of air density it is necessary for to vary inversely with the air density. The value of p=the earphone open circuit R. M. S. pressure (force available at diaphragm divided by effective diaphragm area);

x=the R. M. S. value of the sound wave volume displacement;

Z=the earphone internal acoustic impedance; and

Se=the ear cavity acoustic stiffness PC2 Ve For condition (l), the acoustic volume displacement (x) is represented by the expression:

With (E.) constant (5cl) varies inversely with (p), the air density.

For condition (2), the displacement is p7 :p z 1902+922 p (k) Va Va where: (k) :a constant determined by the ear volume and the predominating acoustic cavity impedance, Vc=the I7=R- M. S. force on the coil winding, in dynes;

Bzffux density in the gap, in gauss;

l :length of wire in gap, in cm.;

i=R. M. S. current in wire, in abamps;

Mt=cffective diaphragm and coil wire mass, in gms.;

Se=effective stiffness of diaphragm, in dynes/cm.;

A :effective diaphragm area, in cm?,

.Se-:acoustic stiffness at ear, in dynes/cm.5; and

Sc=the controlling acoustic cavity stiffness behind the diaphragm, in dynes/cm.5

rom Figure 5 it is seen that the stiffness of the diaphragm, the refiected stiffness of the back cavity, and the refiected stiffness of the ear cavity divide the available force at frequencies below the system resonance.

It is found that with diaphragm materials available today, it is not considered possible to construct a practical eaiphone in which the ear cavity reflected stiffness (A2Se) is very large compared to the diaphragm mechanical stiffness (Sa), the necessary condition for the zero impedance earphone generator specified in the first mentioned condition. Consequently, for the earphone to have constant sensitivity independent of air density, it is essential that t'ne acoustic cavity behind the diaphragm have a reflected stiffness magnitude (AZSC) very much greater than the diaphragm mechanical stiffness (Sd). If this relationship exists the earphone internal impedance will be governed by the back cavity stiffness.

The size of the controlling or back cavity volume is a function of the effective diaphragm area and the stiffness of the diaphragm. The diaphragm displacement is inversely proportional to the stiffness of the back cavity volume so that basic earphone sensitivity is also inversely proportional to the stiffness of the back cavity volume. Since the reflected baclt cavity stiffness must be very much greater than the diaphragm stiffness, it is essential in the design of an earphone to first choose a diaphragm of maximum area and minimum stiffness. Having designed the diaphragm and measured the effective area and stiffness thereof, it is then possible to determine (Sa), the magnitude of the acoustic stiffness required at the maximum density to be encountered, for a preselected earphone sensitivity loss in any range of fiuid medium densities from the equation:

fila

where Figure 6 shows the sensitivity loss in decibels of Aan acoustic stiffness controlled earphone as a function of the rati@ Of thc '.Icccted acoustic stiffness -(Sa=A2.S'a) to diaphragm stiffness (Sd) at maximum density. A family of curves for varying ratios of maximum to minimum air density is plotted wherein the ordinate is the value of 20 login L as determined from the relationship Curve A is a logarithmic curve of this relationship where p. e ual 2 (p') q s Curve B is a logarithmic curve of this relationship where Y `equals Curve C is a logarithmic curve of this relationship where 20 10g10 l0g10 is necessary for a maximum sensitivity loss of l` db over a density variation of about 5/ 1.

Knowing the allowable sensitivity loss and operating range of air densities a value for SQ) l Sd i is obtained. The volume of the back cavity is then determined from the following expression:

V. 1 7 metan-m where Vc=back cavity volume in cubic centimeter;

'g :value determined from Figure 6;

Sd=mechanical diaphragm stilfnessin dynes/cm.;

A=effective diaphragm area in cm.2;

p=maximum operating air density in gms/cm;

C: velocity 0f sound in crm/sec., assumed to be constant for the operating range of air densities; and

V,=the volume of the ear cavity including the space dened by a close fitting earcap and assumed to be 6 cubic centimeters.

Where:

S =the total reflected acoustic stiffness at the mini mum air density, e. p S a; and

M =the total eective mass of the vibrating system.

The value for the upper operating frequency, at the minimum air density, for the acoustic stiffness earphone shown in Figure 6 of the drawing, with a dissipative element added in series with the ear, is proportional to (fo). Consequently, it may be necessary to increase (Sa) beyond the value determined from Figure 6 to obtain sucient frequency range for the device. It is important that (Sd) not be increased to extend the upper operating frequency since this change would reduce causing an undesired sensitivity loss with varying air density.

For a dynamic earphone, acoustic stiffness controlled, the sensitivity may be expressed as follows:

where P.=R. M. S. value of pressure developed in ear cavity in dynes/cm;

P1=electrical power into earphone, assumed equal to 1 milliwatt;

B=iiux density in air gap in gauss;

S.=acoustic stiffness of ear cavity;

A=elective diaphragm area in cm;

fr=earphone fundamental resonance, in cycles/sec.

kw=voice coil Wire resistivity in c. g. s. units;

p,=voice coil Wire density in gm./em.3;

pd=diaphragm density in gm./crn.3;

td=diaphragm thickness in cm.;

cd=a constant determined from the relation (Md= pdtadl);

et-11% (note: the maximuml value for ,=efective mass of voice coil in gms.; and M rl=eective mass of diaphragm in gms.;

Maximum sensitivity for a fixed operating frequency range for the earphone is then obtained if the following conditions, as determined from Equation 9, are satisfied,

1. The coil wire should have a minimum resistivitydensity product.

2. The density and thickness of the diaphragm should be a minimum.

3. The diaphragm effective area should be as large as possible.

4..After designing an optimum diaphragm, the mass of the coil wire should be equal to the mass of the diaphragm.

5. The flux density in the air gap should be as high as possible.

From the explanation outlined above, it will be observed that it is feasible to provide a dynamic earphone having an acoustic stiffness control chamber of such magnitude as to constitute susbtantially the sole acoustic control over diaphragm motion of the earphone so that a minimum loss in sensitivity (within l db) is obtained with changes in altitude from sea level to 40,000 feet.

A preferred embodiment of the present invention is illustrated in Figures 1 and 2 of the drawing. In this particular embodiment, the earphone 1 comprises a hous ing 3 of a design suitable for use in a headset or aviators helmet and a dynamic sound translating unit S'mounted within the housing.

The sound translating unit 5 comprises a magnetic eld structure 7, and a'vibratory system including a vibratory member or diaphragm 9 and a voice coil 10 attached thereto. The magnetic field structure 7 is of conventional design and comprises a cup-shaped eld yoke 11, an annular outer pole piece or plate member 13, `a permanent magnet core 15 and an inner, cylindrical pole piece 17. The magnet core 1.5 and the inner pole piece 17 are coaxially mounted and secured by suitable means in end-to-end relation. The outer pole piece 13 is provided with a central, circular aperture 19 and is mounted across the open end of the eld yoke 11. The magnet core 15 and inner pole piece are concentrically mounted within the yoke 11 with the inner pole piece disposed within the aperture 19 of the outer pole piece 13 in spaced relation to the outer pole piece so as to provide an annular air gap 21 therebetween.

The diaphragm 9 is made from lightweight material such as aluminum and comprises a disc-like member supported at its periphery between a pair of annular spacing rings 23 of nonmagnetic material. The voice coil is attached to the central dome portion 27 of the diaphragm 9. The diaphragm 9 is mounted on the outer pole piece 13 with the voice coil 10 freely disposed within the air gap 21. The diaphragm 9 and voice coil 10 are arranged for simultaneous vibratory movement and serve to convert electrical signals fed into the voice coil from an external source into corresponding acoustic Waves generated by the diaphragm, in a manner well known in the art.

An annular seal ring 28 of non-magnetic material is disposed in close-fitting relation around the inner pole piece 17. The ring 28 is arranged to extend across the air gap 21 and abut the outer pole piece 13 so that an entirely closed cavity 29 is provided behind the diaphragm 9. An air pressure equalization opening or restricted passageway 30 is disposed on the inner periphery of the seal ring for connecting the cavity 29 with the fluid medium within which the earphone is being operated. The dimensions of the opening 30 are such as to provide a high impedance to sound waves within` the useful operating audio frequency range of the earphone. Thus, the cavity 29 defined by the diaphragm 9, the outer pole piece 13, the inner pole piece 17 and the seal ring 28 is acoustically sealed. It may be noted, howeverthat the pressure equalization opening 30 may be provided at any convenient location in the magnetic field structure which will connect the cavity 29 with the fluid medium in which'the earphone is being operated. Although an opening is provided as a means for equalizing pressure, other suitable arrangements may be found desirable.

The sound translating unit 5 is mounted in any convenient manner which will securely hold it in the housing 3. In order to securely fasten the translating unit in the housing against movement, a spring 31 is disposed between the field yoke 11 and the back of the housing 33. The housing 3 is also provided with a protective cover 35 mounted in spaced relation to and in front of the diaphragm 9. A plurality of apertures 37 are provided in the cover 35 for the purpose of transmitting sound waves generated by the diaphragm to the ambient or, when the earphone is worn by a user thereof, to the cavity provided by the earcap (not shown) for the earphone. ing the earphone components against dirt particles is provided for the apertures 37 which also may be ernployed to serve as a damping medium to provide control of frequency response beyond the control compensated for by the stiffness control cavity behind the diaphragm.

To facilitate an understanding of the elements of the structure embodying the present invention, as illust-rated in Figures l and 2 of the drawing, reference may be had to the electrical analogue thereof shown in Figure 7 of the drawing, wherein:

F=the R. M. S. value of the force on the diaphragm,

in dynes;

B=flux density in the air gap, in gauss;

lzlength .of coil wire in the voice coil, in centirnetersI -i=R. M. S. valuevof the current in the voice coiljin abamps;

Mt=the effective diaphragm and coil mass, in grams;

A2Sb=reflected acoustic stiffness of the chamber behind the diaphragm, in dynes/cm.;

A2Sfc=reilected acoustic stiffness of the chamber in front of the diaphragm, in dynes/cm.;

A2Ra=reilected acoustic damping resistance in the earcap openings, in dynes/cm./sec.; and

A2Sg=reilected acoustic stiffness of the chamber formed A screen 39 of a mesh suitable for protect- A between the earcap and the head of the user, in dynes/cm.

One particular embodiment which was constructed and successfully operated comprised a dural diaphragm of .001 inch thickness having an effective area of approximately 6 cm.2, an effective mass of approximately 60 milligrams, and an effective stiffness of approximately 4.5 106 dynes/cm. An aluminum, edge-wound, selfsupporting voice coil having an effective mass of about 60 milligrams was supported from the diaphragm and disposed in an air gap having a magnetic field of 9000 gauss. An acoustically closed chamber was provided behind the diaphragm of a design suitable to provide a volume as determined Vby Equation 8 above. This volume was determined to be .6 cc. in order to maintain a ratio of reflected acoustic stiffness to diaphragm stiffness of about 35 to l. The earphone resonant frequency determined from Equation 8, and verified experimentally, was about 5,000 cycles per second. The earphone sensitivity was 24 dynes/ cm. output pressure in a 6 cc. coupler for 1 milliwatt of electrical input power which approximated the value calculated from Equation 9. Figure 8 of the drawing represents frequency response curves of this particular earphone measured on a 6 cc. American Standards Association coupler taken at densities corresponding to sea level and 40,000 feet altitude pressures. As observed from Figure 8, curve A, which represents response at sea level, is only approximately l db greater in sensitivity from the response shown by curve B taken at a density equivalent to that encountered at 40,000 feet altitude. A comparison of the curves shown in Figure 8 with a similar set of curves, shown in Figure 9 of the drawing and representing the frequency response of a conventional type earphone without compensation for sensitivity changes taken under similar conditions, demonstrates the improvement provided by the present invention.

From the foregoing, it is apparent that the present invention provides an acoustic control which provides substantially uniform sensitivity for a dynamic earphone operated in a fluid medium in which there exists substantial differences in density.

Although but a single, preferred embodiment of the present invention is illustrated and described herein, it should be obvious to those persons skilled in the art Vthat various changes and modifications are possible within the spirit of the invention. For example, the use of a stiff ness control cavity in back of the diaphragm is not limited in its application to a dynamic earphone. It is applicable to other types of earphones such, for example, as a magnetic earphone, a crystal earphone, or a balanced armature type. It may also be applicable to microphones, loudspeakers or other sound translating devices. Therefore, it is desired that the particular form of the present invention described herein shall be considered as illustrative and not as limiting.

What is claimed is:

l. In aV sound translating device for use in a fluid medium having relatively great variations in density and including a magnetic field structure, a Avibratory system comprising a diaphragm mounted for vibratory movement, a voice coil attached to said diaphragm for movement therewith, the mass of said voice coil being equal to the mass of said diaphragm, said diaphragm and said magnetic field structure cooperating to provide an acoustically closed cavity for controlling diaphragm movement thereby to obtain maximum sensitivity over the ranges of density of said fluid medium, and means for equalizing the density of the medium, said last mentioned means including a restricted passageway connecting said cavity with the fluid medium within which said device is disposed, said passageway being restricted so as to provide a high impedance to audio frequency sound waves.

2. In a sound translating .device for use in a fluid medium having relatively great variations in density, the

L s :(P' L) L-1 A2 wherein p is the maximum air density, p is the minimum air density,

has a value at least 2,

L is the ratio of sound pressure in the ear cavity at maximum air density to the sound pressure at reduced air density, this ratio having a value not greater than 1.4,

Sd is the elective stiffness of the diaphragm, and

A is the area of the diaphragm.

3. In a sound translating device for use in a uid medium having relatively great variations in density, a vibratory system for said device comprising a member mounted for vibratory movement, means defining with said member an acoustically closed chamber, the ratio of the reected acoustic stiiness of said chamber to the mechanical acoustic stiiness of said member being within the range of 8 to 1 and 35 to 1 such that movement of said member is controlled by the acoustic stiffness of said chamber over the range of densities in said uid medium, and means for equalizing the density of the medium within which said device is disposed.

4. In a headset receiver including an earcap defining a cavity about the ear of a user, said receiver being intended for use in a uid medium having relatively great variations in density, a vibratory system for said receiver comprising a diaphragm mounted for vibratory movement, means defining with said diaphragm an acoustically closed cavity, and means for equalizing the density of the fluid within said chamber with the density of the uid within said ear cavity, the volume of said closed cavity having a magnitude found from the equation 10 wherein Sl Si has a Value between 8 and 35,

Sd is the effective stiiness of the diaphragm,

A is the eiective diaphragm area,

p is the maximum operating air density,

C' is the velocity of sound and,

V. is the volume of the ear cavity including the space defined by a close fitting earcap.

5. The invention as defined in claim 4 wherein said equalizing means comprises an opening in said magnetic iield structure connecting said cavity with the medium within which said device is disposed, said opening being dimensioned to provide high impedance to audio frequency sound waves.

6. The invention as dened in claim 4 wherein said equalizing means comprises a restricted passageway connecting said cavity with the uid medium within which said device is disposed, said passageway being restricted so as to provide a high impedance to audio frequency sound waves.

7. In a headset receiver of the type useful in a uid medium having relatively great differences in density, the combination with a magnetic field structure and a dia phragm mounted for vibratory movement of means including said magnetic eld structure deiining an acoustically closed cavity behind said diaphragm, the ratio of the reflected acoustic stiffness of said chamber to the mechanical acoustic stitness of said diaphragm being within the range of 8 to 1 and 35 to 1 whereby the sensitivity of said receiver over the operating frequency range thereof is substantially uniform for all densities in which said receiver is operated in said iuid medium, and means impervious to the passage of acoustic sound waves for equalizing the density of the iluid within said cavity with the density of the uid medium within which said rcceiver is disposed.

References Cited in the le of this patent UNITED STATES PATENTS 1,766,473 Wente June 24, 1930 1,847,702 Thuras Mar. l, 1932 2,162,270 Mott .Tune 13, 1939 2,238,741 Lauier Apr. 15, 1941 2,429,104 Olson Oct. 14, 1947 2,434,900 Black, Ir., et al Jan. 27, 1948 2,505,519 Beranek Apr. 25, 1950 2,509,224 Gayford May 30, 1950 

